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Data Communications
Computer Line
Control Mode
Terminal
POLL
Sequence
No response (Terminal
does not recognize
POLL sequence as
valid for this terminal
EOT
(Terminal has no
data to send in
response to POLL)
Text Block (Ter-
minal responds
to POLL by
transmitting text
terminal goes in-
to Text-Out mode)
STX. GID. DID. CN . ETX
Terminal •
key
has been pressed
Select
Sequence
No response (Ter-
minal does not
recognize SELECT
sequence as valid for
this terminal)
WACK
(Terminal is tem-
porarily not able
to accept Text
because of lack
of buffer. space)
ACKO
(Terminal ack-
nowledges that it
is ready to accept
Text terminal goes
into Text-In mode)
RVI
(Terminal acknowledges
that it is ready to accept
text, b u t .
key has been pressed,
terminal goes into Text-
In mode.)
Text-Out Mode
goes Into CONTROL mode)
send
Computer
Line
Terminal
+
+
Text block
ACKOorACK1
EOT
(Computer decides
(Computer
to abort current terminal
acknowledges
operation)
Text block as
HI
-
Terminal stops operation and goes into
good)
-
NAK
(Computer detects bad VRC
CONTROL mode
or invalid block check character)
"
" I
-
Terminal retransmits last Text block
-
RVI (Computer acknowledges Text
uter
block as good but tells terminal to stop)
wledges
I
-
Terminal sends EOT and goes into
xt block
-
lis ter-
CONTROL mode
that
ter has
-
Terminal transmits next Text block. If all
re buffer
-
Text blocks transmitted, terminal sends
for add'i
EOT and goes into CONTROL mode
I
-
Terminal sends ENQ
-
-
t
+
- ENQ
eply
EOT
(Computer decides to abort
(Terminal response to
WACK)
uter
current terminal operation)
smits its
I
-
Terminal stops operation and goes into
us reply)
-
CONTROL mode
-
Terminal responds in one of the ways
-
listed above (dependent on content of
_ computer's last reply)
+
+
-
TID
NAK
(Computer acknowledges
(Terminal is temporarily out of data)
puter
delay and is ready for more text)
-
es to
-
Terminal responds in one of the ways
current
listed above
·rloperation.)
-
Terminal stops operation and goes into
-
CONTROL mode
-
,
--
EOT
Computer interprets EOT as the
(All Text blocks completed
terminal having no more Text to
or RVI received. Terminal
EOT
(Com
decid
abort
termin
Last R
(Comp
retran
previo
WACK
(Comp
ackno
last Te
but te
minal
compu
no mo
space
text)
Figure 7-20. Operation of Block Protocol Control Characters
7-42
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